The remains of the prehistoric, Iron Age, Pen-y-Dinas hillfort stand on a prominent peak (230 metres high), overlooking the coastal plain of Dyffryn Ardudwy. Its shape is oval, following the shape of the hill that it stands upon, and it measures approximately 60 metres by 50 metres. The wall of the hillfort is better preserved on the west side, which is where the entrance to the fort used to be. There is evidence of a ditch around the hillfort, and a possible second outer rampart on the west side, which would have enclosed a small annex.
An excavation was conducted at the site in 1919, when pieces of coarse pottery and a piece of iron were recovered, but nothing of a datable age. In the vicinity of Pen-y-Dinas are the remains of Medieval settlements, and it is thought that the stone walls were dismantled to build the dry stone walls during this period.